The present invention relates to alkyl-substituted diisocyanates in the form of a mixture of homologues and isomers. These alkyl-substituted diisocyanates are useful structural components in the preparation of polyurethane plastic materials by the isocyanate polyaddition process.
Aromatic diisocyanates are known (cf "Polyurethanes", Chemistry and Technology, Part I, by Saunders and Frisch, Interscience Publishers, (1962), pages 17 et seq.). In particular, 2,4- and 2,6-diisocyanato toluenes and mixtures of these isomers ("TDI") and 2,4'- and 4,4'-diisocyanato diphenyl methane and mixtures of these isomers, as well as mixtures of these last-mentioned diisocyanates with high homologues of the type obtained by phosgenation of aniline/formaldehyde condensates ("MDI") are of great commercial importance. The diisocyanato toluenes which are liquid at room temperature have the disadvantage of having a relatively high vapor pressure which makes it necessary to take comprehensive measures during processing in order to keep the TDI concentrations in the air within permissible limits. Another disadvantage of TDI is that the diisocyanato toluene isomers have melting points which are only slightly below room temperature so that at least partial crystallization of the diisocyanate or of the diisocyanate mixtures occurs during storage and during transportation of TDI in cold weather. Such partial crystallization makes it necessary to subject the diisocyanate or the diisocyanate mixtures to a melting process prior to use in polyurethane production. Moreover, although the above-mentioned diisocyanates of the diphenyl methane series have a far lower vapor pressure than TDI, which makes their processing far less problematical from the physiological point of view, these polyisocyanates have the disadvantage that the 4,4'-diisocyanato diphenyl methane present in them as main component is a solid substance at room temperature. These polyisocyanates must, therefore, be melted or liquefied by a chemical reaction before further processing.
U.S. Pat. No. 2,934,571 discloses preparation of long-chained dinitro alkyl benzenes. U.S. Pat. No. 2,986,576 (corresponding to German Auslegeschrift No. 1,123,662) teaches that such dinitro alkyl benzenes can be used to produce the corresponding diamines which may then be converted to the corresponding diisocyanates. The alkyl benzenes used as starting materials in U.S. Pat. No. 2,934,571 (particularly in the Examples) are very specific compounds which have a branched oligopropylene side chain. A special nitration process is described for the nitration of these hydrocarbons because conventional nitration processes obviously fail to nitrate these hydrocarbons into the corresponding dinitro alkyl benzenes. In spite of the complex process (which involves the use of sulfuric acid containing sulfur trioxide) undesirable residual mononitro compounds are formed (note Example 2 infra). The further processing of this disclosed intermediate stage into diisocyanates via the corresponding diamines inevitably leads to diisocyanates in which undesirable monoisocyanates are present. Such monoisocyanates act as chain breakers in the preparation of polyurethane plastics by the isocyanate addition process and lead to unusable products (see Example 2 infra). These difficulties make it clear why the diisocyanates in the above-mentioned disclosures have not been used in commercial polyurethane processes.